1. Field of the Invention
The present invention relates to a magnetic tunneling structure, and in particular a magnetic tunneling structure formed of two magnetic layers having an insulating tunneling barrier layer sandwiched therebetween.
2. Background of the Related Art
Magnetic storage technology is currently enjoying a 60% compound annual growth rate, with data rate increases of 30-40% per year. This has enabled the hard disk drive industry to drive storage costs down by approximately 40% per year. This rate of improvement shows no signs of diminishing and promises to continue at the present rate or even to accelerate.
While many factors such as fly height, media, etc. can also be improved, magnetic recording heads capable of higher speeds and densities are the key to continuing this trend. The magnetic sensing used so far, is done by a change in magnetoresistance (MR) induced in the sensing head. Research on magnetoresistance has been particularly active in recent years towards this goal. The materials explored so far exploit the giant magnetoresistance (GMR) effect in heterogeneous magnetic systems, such as layered and granular solids, where the dominant mechanism responsible for GMR is spin-dependent electron scattering, which is greatly enhanced in heterogeneous media. To extend the recording density beyond approximately 10 Gbytes/in2, a current perpendicular to the plane (CPP) mode GMR head was proposed in 1995. GMR materials in this configuration have low electrical impedance complicating their use in disk drives. A second type of magnetoresistance effect occurs in manganese perovskites. These materials display even larger magnetoresistance (MR), named xe2x80x9ccolossalxe2x80x9d magnetoresistance (CMR), but the effects only occur in a large field and have a strong temperature dependence. The potential of CMR for low-field and room-temperature applications has yet to be determined.
Another promising source of large magnetoresistance effects has been magnetic tunnel junctions (MTJs). Magnetic tunneling structures generally have two magnetic electrodes with different coercivities separated by a very thin insulating layer. A tunneling effect manifests depending upon the relative angles of magnetization of the two ferromagnetic layers. Since the directions of the magnetizations can be altered by an external field, the tunneling resistance is sensitive to the field. According to known theories, a range of field exists where the spins in both electrodes are antiparallel and the tunneling resistance as a function of the magnetic field will be larger. For all other field values, the spins in both electrodes are parallel and the resistance will have a lower value.
Although MTJ structures have been studied for more than twenty years, it is only recently that significant changes in MR (xcx9c20-30% at room temperature) have been observed, for absolute resistance values in the 102 kilo-ohm range for micron-size devices. All the structures studied that have yielded high MR values at room temperature, consist of polycrystalline magnetic layers separated by a thin aluminum oxide insulating layer. Different magnetic materials and/or shapes have been employed to create the two different anisotropys in some cases. Other devices have employed an antiferromagnetic layer to pin one of the ferromagnetic layers.
For example, U.S. Pat. No. 5,835,314 to Moodera, which is hereby incorporated by reference, discloses a magnetic tunneling junction 20 comprising a substrate 22, a seeding layer 24, a first ferromagnetic layer 12, an insulating tunnel barrier layer 14, and a second ferromagnetic layer 10. The first ferromagnetic material layer is formed of Cobalt Iron and the second ferromagnetic material layer is formed of either Cobalt or Nickel Iron. The insulating tunnel barrier layer is formed of Aluminum Oxide or other nitrides.
U.S. Pat. No. 5,953,248 to Chen, which is hereby incorporated by reference, discloses magnetic tunneling injunction 10 comprising a supporting substrate 11, a magnetoresistive structure 12 supported on the substrate, an electrically insulating material layer 13 positioned on the structure 12 and a magnetoresistive structure 15 positioned on the electrically insulating material layer 13. The magnetoresistive structure 15 comprises an antiferromagnetically coupled multi-layer structure including magnetoresistive layers 17 and 18 having a nonmagnetic conductive layer 19 situated in parallel juxtaposition between the magnetoresistive layers 17 and 18. The magnetoresistive layers 17 and 18 may be single layers of ferromagnetic material such as a layer of Nickel, Iron, Cobalt, or alloys thereof, or alternatively, either of layers 17 and 18 can be a composite ferromagnetic layer, such a layer of Nickel-Iron-Cobalt covering a layer of Cobalt-Iron or three layer structures including layers of Cobalt-Iron and Nickel-Iron-Cobalt and Cobalt-Iron with Cobalt-Iron at the interface with adjacent layers. The magnetoresistive structure 12 is similar to the structure 15 and includes magnetoresistive layers 25 and 26 separated by a non-magnetic conducting layer 27. Chen notes that only the layers 17 and 26, adjacent to the electrically insulating material layer 13, contribute to the magnetoresistance of the magnetic tunneling injunction 10. Chen teaches Aluminum Oxide as an example of the electrically insulating material layer 13.
U.S. Pat. No. 5,793,697 to Scheurelein and U.S. Pat. No. 5,640,343 to Gallagher, which are hereby incorporated by reference, each discloses a magnetic tunneling junction comprising a template layer 15, such as Pt, a initial ferromagnetic layer 16, such as Permalloy (Nixe2x80x94Fe), an antiferromagnetic layer 18, such as Mnxe2x80x94Fe, a fixed ferromagnetic layer 10, such a Coxe2x80x94Fe or Permalloy, a thin tunneling barrier layer 22 of Alumna (Al2O3), a soft ferromagnetic layer 24, such as a sandwich of thin Coxe2x80x94Fe with Permalloy, and a contact layer 25, such as Pt.
An object of the invention is to solve at least the problems and/or disadvantages associated with prior art devices and to provide at least the advantages described hereinafter.
It is an object of the invention to provide a novel magnetic tunneling structure.
It is another object of the invention to provide a magnetic tunneling structure with improved magnetoresistive effect in comparison to conventional magnetic tunneling structures.
It is a still further object of the invention to provide a magnetic tunneling structure with lower absolute electrical impedance in comparison to conventional magnetic tunneling structures.
It is an additional object of the invention to provide a magnetic tunneling structure with a higher degree of polarization of the ferromagnetic layers in comparison to conventional magnetic tunneling structures.
It is another object of the invention to provide a magnetic tunneling structure with a lower tunneling barrier in comparison to conventional magnetic tunneling structures.
It is yet another object of the invention to provide a magnetic tunneling structure reduced in size in comparison to conventional magnetic tunneling structures.
It is a further object of the invention to provide a magnetic tunneling structure having an oxide-free tunneling barrier.
It is a further object to provide a magnetic tunneling structure with reduced oxidation between the ferromagnetic layers and the insulating tunneling barrier layer.
To achieve the above objects, a magnetic tunneling structure is provided that comprises, first and second ferromagnetic layers and an insulating tunneling barrier layer disposed between the first and second ferromagnetic layers, wherein the first ferromagnetic layer is a single crystalline layer and the second ferromagnetic layer is a polycrystalline layer. The single crystalline layer and the polycrystalline layer work in combination to provide two states of magnetization. The multilayer structure may be disposed on a substrate, for example, silicon; however, other materials may also be appropriate.
Further, the insulating tunneling barrier layer is preferably formed of a nitride, for example, boron nitride; however, other materials may also be appropriate. The boron nitride layer is preferably grown on the first ferromagnetic layer, preferably using electron cyclotron resonance-assisted sputtering.
The first and second ferromagnetic layers are preferably formed of the same ferromagnetic material, for example, nickel (Ni), cobalt (Co) or iron (Fe). More preferably, the first ferromagnetic layer is formed of a fcc Co single crystalline layer, while the second ferromagnetic layer is formed of a hcp Co polycrystalline layer. The first ferromagnetic layer may be grown on a copper (Cu) fcc buffer layer or on a single crystal MgO substrate. The second ferromagnetic layer may be grown on the insulating tunneling barrier layer.
To achieve the above objects, a magnetic tunneling structure is also provided comprising first and second ferromagnetic layers and an insulating tunneling barrier layer disposed between the first and second ferromagnetic layers, wherein the insulating tunneling barrier layer is a nitride layer grown directly on the first ferromagnetic layer. The insulating tunneling barrier layer is preferably a boron nitride layer.
The first and second ferromagnetic layers are preferably formed of the same ferromagnetic material, for example, Ni, Co or Fe, but have different crystallographic structures. The first ferromagnetic layer is preferably a single crystalline layer, and the second ferromagnetic layer is preferably a polycrystalline layer. More preferably, the first ferromagnetic layer is formed of a fcc Co single crystalline layer, while the second ferromagnetic layer is formed of a hcp Co polycrystalline layer. The nitride layer is preferably grown on the first ferromagnetic layer, preferably using electron cyclotron resonance-assisted sputtering.
To achieve the above objects, a magnetic tunneling structure is also provided comprising first and second ferromagnetic layers and an insulating tunneling barrier layer disposed between the first and second ferromagnetic layers, wherein the first and second ferromagnetic layers are formed of the same metal, but have different crystallographic structures. Preferably, the first ferromagnetic layer is a single crystalline layer and the second ferromagnetic layer is a polycrystalline layer.
Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims.